Fireball is one of two ways that can be used to describe how an unexplainable event in a Tiradesverse movie occurred. (The other is orbital wobble. The hosts used to use "It worked in Blazing Saddles!", as well, but it was retired after Transformers.) The joke is that there is no explanation given at all. All that is said is "Fireball" and the hosts move on.
The origin of this is the movie, Street Fighter: The Legend of Chun-Li where Chun-Li is able to create a fireball with no explanation as to how.
This is similar to "Because fuck you, that's why."